The invention relates to communication systems and, more particularly, to noise suppression of transmitted voice signals.
In a communications system, a transmitting station may employ a noise suppression mechanism in order to reduce the noise content of a transmitted voice signal. This can be particularly useful when the transmitting station is a mobile handset or hands-free telephone operating in the presence of background noise. In these environments, a sudden increase in background noise can cause a far-end listener to hear an undesirable level of noise. This problem is particularly apparent when the transmitter station is operating as a mobile station and the transmitter station includes noise suppression technology. While current noise suppression techniques are effective in reducing background noise in a static or slowly changing noise environment, noise suppression performance can be significantly degraded when the transmitting station is operated in the presence of a rapidly changing noise environment.
In mobile environments, large changes in background noise can be brought about when the user of the mobile transmitter activates a fan, lowers a window while the mobile station is in motion, or is otherwise subjected to significant and sudden changes in the background noise within the mobile station. The background noise within the mobile unit can also be affected by numerous other changes within the mobile station.
In typical mobile transmitters which use voice activity detection internal to a noise suppression algorithm, an increase in background noise can be interpreted by the noise suppression algorithm as a voice signal from the user of the mobile transmitter. This condition is brought about due to the inter-dependency between the voice activity detection and the noise floor estimate computed by the noise suppression algorithm. One noise suppression technique, such as a stationary spectral check, has been used with some success in order to mitigate be effects of sudden increases in background noise. However, in practice, this solution has been shown to be inadequate in many cases due to the time required for the noise suppression algorithm to reduce the background noise to an acceptable level. In some cases, this time period can be 10-20 seconds in duration. In other cases, the system can experience a locked fault condition in which noise floor updates cease to occur. This results in the transmitter being placed in a condition where the listener is subjected to an unacceptable amount of noise for an extended period of time.
Therefore, it is highly desirable for the noise suppression method and system to adapt to sudden increases in background noise through the use of a voice activity detector with reduced inter-dependency between voice activity detection and noise floor estimates. Such a system would provide a capability for lower noise transmissions while a mobile station is operating in the presence of widely varying background noise.